1. Field of the Invention
The present invention relates to an electrostatic protector for protecting an input circuit or an output circuit of a semiconductor integrated circuit, and more particularly, to a circuit for protecting a base-emitter junction of a transistor against an incoming surge voltage.
2. Description of the Related Art
An electrostatic protector is installed in an output circuit of a semiconductor integrated circuit, such as is shown in FIG. 5 or FIG. 6, or in an input circuit such as that shown in FIG. 7. In the output circuit of the integrated circuit (IC) of FIG. 5, a collector-emitter path of an output NPN transistor Q.sub.1 and a resistor R are connected in series between a Vcc power source line (referred to as a Vcc line) and a ground potential line (referred to as a ground line). The emitter of the transistor Q.sub.1 is connected to an output terminal OUT. A diode D.sub.1, for absorbing a positive surge current, is connected between the emitter of the transistor Q.sub.1 and the Vcc line, while a diode D.sub.2, for absorbing a negative surge current, is connected between the emitter of the transistor Q.sub.1 and the ground line.
The diodes D.sub.1 and D.sub.2 of this protector respectively provide electrostatic protection for the junction between the base and emitter of the transistor Q.sub.1 against positive and negative surge voltages coming in through the output terminal OUT.
In the output circuit of FIG. 6, a collector-emitter path of an output NPN transistor Q.sub.1 and a resistor R are connected in series between a Vcc power source line (referred to as a Vcc or power line) and a ground potential line (referred to as a ground line). In this circuit, resistor R is P type diffusion resistor, and formed in N type resistor island, and the voltage potential Vcc is supplied to the resistor island. Therefore, a surge diode structure D.sub.1-2 is equivalently formed between resistor R and Vcc. The emitter of the transistor Q.sub.1 is connected to an output terminal OUT, and a diode D.sub.2, for absorbing a negative surge current, is connected between the emitter of the transistor Q.sub.1 and the ground line.
In the input circuit of FIG. 7, a resistor R is connected between the emitter of an input NPN transistor Q.sub.2 and a ground line. The base of the transistor Q.sub.2 is connected to an input terminal IN, and the collector thereof is connected to an internal circuit. A diode D.sub.1, for absorbing a positive surge current, is connected between the base of the NPN transistor Q.sub.2 and an Vcc line, while a diode D.sub.2, for absorbing a negative surge current, is connected between the base of the transistor Q.sub.2 and the ground line.
In the case of the above protector, the diodes D.sub.1 and D.sub.2 respectively provide electrostatic protection for the junction between the base and emitter of the transistor Q.sub.2 against positive and negative surge voltages coming in through the input terminal IN.
Let us consider in detail a case where a surge voltage arrives at the output terminal OUT or the input terminal IN. If the surge voltage is negative, the ground line always exhibits a low impedance, and therefore a negative surge current caused by the negative surge voltage passes through the diode D.sub.2. In other words, the negative surge current is absorbed by the diode D.sub.2. If, on the other hand, the surge voltage is positive, the Vcc line exhibits a low impedance, a positive surge current caused by the positive surge voltage passes through the diode D.sub.1 or the equivalent diode D.sub.1-2. In other words, the positive surge current is absorbed by the diode D.sub.1 or D.sub.1-2. When the Vcc line is open, or in a high impedance state, a surge current absorbing path including the diode D.sub.1 or D.sub.1-2 is open, in which case the surge voltage, when positive, will destroy the base-emitter junction of the transistor Q.sub.1 or Q.sub.2.
Thus, in the case of a positive surge voltage, the electrostatic protecting circuit to which the present invention is not embodied fails to absorb the positive surge current by the related diode, when the Vcc or power line is in a high impedance state. Accordingly, the base-emitter junction of the NPN transistor will be possibly destroyed.